Process of refining raw cellulose pulp



July 28,

Filed may 15, 1929 Patented July' 28,1931

yUNF-ran STATES `PROCESS F REFINING Application tiled May 15,

This invention relates to a process ofvrefining .raw or unbleached cellulose pulps such as kraft, soda, .or sulphite,- its object being the production of a White product suitable for use as a high grade raw material for papermaking and for the preparation of cellulose derivatives. f

Raw cellulose pulps resulting from the various processes of fiber liberation vary considerably, their physical vcharacteristics and compositions depending upon the fiberliberating 4process by which' they were roduced and the conditions under which ber liberation was effected. When such pulps are produced from raw cellulosic material such as Wood, they contain a substantial amount of ligneous andother coloring matter. and less resistant celluloses, the usual kraft pulp being quite high in lignin and 2o accordingly requiring such a large amount of bleach to be Whitened as to undergo physical and chemical degradation during the bleaching operation. Ithas been found that if ravv pulp such askraft is treated with lignin-reactive reagents, such as chlorine.

Water or hypochlorite or bisulphite liquors, to effect a thorough reaction .onthe ligneous groups in the pulp, then washed free 'of reagent and soluble reaction products, and then digested in an `alkaline liquor of -suitable strength and under suitable temperature conditions, the resulting product isV readily bleachble to whiteness without-injury to the pulp. The lignin-reactive reagents mentioned evidently react on the ligneous groups presentin the pulpto produce water-soluble products, which are removed during washing, and water-insoluble prodf ucts, which are soluble in alkaline liquors.

The alkaline liquor, depending on its alkalinity and temperature, may serve not only .to remove Water-insoluble 'acidic products, but also less'V resistant celluloses 'and .other non-alpha cellulose impurities present in the ul I.) Iphave found that raw pul particularly after treatment with the ignin-reactive reagents described contains non-.alpha cellulose components wliich are readily soluble in liquors of comparatively i lowv alkalinity,

PATENT o1-F1ca- GEoReE A. RICHTER, oF BERLIN', NEW HAMPSHIRE AssIeNoR 'ro ERowN COMPANY,4 0F BERLIN, NEW HAMPSHIRE, A CORPORATION oF MAINE EAW oELLULosE PULP 1929. Serial No. 363,158.

and non-alpha cellulose components whichl high alkalinity only. In accordance wit the process of the present invention, therefore, raw .pulp which has been treated with vsuch lignin-reactive reagents is first digested inan alkaline liquor o f 4comparatively low alkalinity, andv then in a liquor of comparatively high alkalinity. The initial alkaline liquor may be a sodium carbonate isolution or a milk of lime suspension, as, I have found that such liquors are capable of readily removing water-insoluble reaction products formed Aduring the previous treatment lwith lignin-reactive reagent. lHeretofore, it has been proposed to digest. pulp which has previously been treated with a lignin-reactive reagent such; as chlorine Water, in alkaline liquors of comparatively high alkalinity,for instance, solutions of caustic soda and/or sodium sulphide, with the result that reaction products present' in the ulp after-'washing consumed a substantlal amount of such alkali. Such reaction products may, however, be removed with sodium carbonate and/or a milk of lime liquor, whereupon the more difcultly soluble non-al ha cellulose components in the pulp may e removed by -digesting the pulp insolutions of stronger alkalies,'such as caustic soda and/or sodium sulphide. In a, kraft mill, the'process of the present in- .vention makes possible a saving of lime, as sodium carbonate v is produced in large quantity duri-ng the smelting of thev inoranic content vof the spent cooking liquor.k

he sodium carbonate occurs in the smelt in admixture with sodium sulphide, but it may be readily removed from the sodium `sul hide as' by fractional crystallization, an used in the preparation -of an alkaline' liquor for initially dlgesting the pulp, thereby efecting a saving of the lime customarily used .in causticizing such sodium carbonate.

The mother liquor, which is rich in sodium sulphide, aftercausticization of residual sodium carbonate therein, if desired, may be used in preparing the liquor for the second alkaline digestion of the pulp.- When a concentrate liquor at 'comparatively low alkali, if desired, orthe preparation of a cooking liquor for liberating fiber from raw wood.

The accompanying drawing represents a flow sheet in which the steps of the process of the present invention are interrelated with a process of producing kraft pulp.

Referring to the drawing, 1 represents a digester in which the cooking of raw cellulosic material such as chipped wood may be effected in a 'kraft'cooking liquor contain-v ing, say, 2 to 2.4 poundsof active alkali, calculated as Na2O per cubic foot, and 40% or higher sulphidity. Cooking may be carried out for a four-hour period at a maximum temperature of 3350 F. The liberated pulp may `be delivered into a washer 2, the washed pulp being passed into a treating tank 3, and the cooking liquor being passed into an evaporator 4. The pul may be treated with `one or more lignin-reactive reagents. For instance, the pulp may be treated at a consistency of from 2% to 8% in chlorine water containing, say, 3% to 8% chlorine, based on dry pulp, the time of contact ranging from 20 to 200 minutes, and the temperature from 50o to 90 F., 'depending upon the characteristics of the raw pulp and the color and other characteristics de` sired in the final product. The higher the chlorine usage, the greater the advantages accruing from the present process, 'as more reaction product is removed in the weak alkaline solution. If desired, a bisulphite liquor may be used in lieu of thechlorine water, the bisulphitereacting to form lignone-sulphonic reaction products, partv of which are soluble in water and part of which are soluble in the weak alkaline solution. The pulp may be treated at a consistency of from 2% to 8% in a boiling bisulphite liquor, preferably a sodium bisulphite liquor containing about 1% free and 1% combined SO2. It may be desirable to combine a chlorine water treatment and a bisulphite liquor treatment, in which lcase it is preferable to chlorinate the stock first, as some of the chlorinated products are soluble in hot bisulphite liquor. After treat# lment with a' lignin-reactive reagent, the

pulp may be delivered into a washer 5, wherein the reagent together with soluble l reaction products is washed from the pulp,

whereupon it may be delivered into a tank 6 for digestion in weak alkaline liquor. 1/2% to 1% sodium carbonate solution at boiling temperature may be used, digestion being continued for abou-LU two to four hours,

\ depending upon the strength of the solution. The sodium carbonate maybe derived fromthe smelt of the inorganic compounds of the spent kraft cooking liquor. As shown on the drawing, after the spent krat liquor has been concentrated in the evaporator L1, it may be delivered into a furnace 7, wherein' its organic content `may be burned and its inorganic content smelted in a reducing atmosphere to produce a sodium carbonatesodium sulphide smelt. The hot smelt may be delivered into a bodyof water in a tan 6, to produce a hot smelt solution, which may be diluted to the desired strength and passed into the tank 6 for the digestion of the pulp, in which case the alkaline liquor consists of a sodium carbonate-sodium sulphide solution. Preferably, however, as indicated in dotted outline, the hot concentrated smelt solution is passed through a cooler 9 designed to cause crystallization of sodium carbonate only to take place, the crystallized sodium carbonate being delivered into the tank 6 to form a solution of the strength desired for thedigestion of the pulp. The mother liquor, rich in sodium sulphide, may be passed into a tank 10, from which it may be withdrawn, preferably after causticization of its residual sodimm,

the mixture of spent liquors being evapoi rated and passed into the furnace 7, asl previously described. The concentration of alkali and temperature durin thel second digestion depends upon the c aracteri'stics desired in the final product. For instance, if it is desired to produce a product of high alpha cellulose content, such digestion may be carried out in a liquor contalning 5% to 6% alkali calculated as NaOI-Iin the -form of NaOH and/or NazS, 'at a temperature of about 20o C. Such a liquor effects a sub. stantial removal of pentosans and less-resistant celluloses from the pulp, the alpha cellulose content of the pulp bein increased correspondingly. The pulp resu ting lfrom such digestion is further characterized by a high strength, tear resistance, folding endurance, and softness. It has a pentosan content ranging from 2% to 5%, as compared with a pentosan contentof 8% to 10% intended primarily for papermaking is de- A sired, as much diluter alkaline liquor may be used at relatively high temperatures, say, 212 F., in which case less pentosans will be removed. When a concentrated liquor is used, the spent cooking liquor has a high 'sodium sulphide.

content of unspent or active alkali. Accordingly, after the pulp has been passed through a washer 13 preparatory to final bleaching, thev liquor1A from the'washer may be used as the cooking liquor in the digester `1, fresh lcaustic soda beingV added to the liquor before such use, if desired. If, however, a comparatively dilute liquor has been employed for such second alkaline digestion, andthe liquor contains little active alkali lafter digestion, itvmay be mixed with the liquors from the washers 11 and 2 into the evaporator 4, whereupon the mixed liquors may be passed into the smelting furnace.

As previously stated, the initial alkaline digestion of the pulp may be effected With a lime liquor in lieu of a sodium carbonate solution. Such a procedure also makes possible a saving, as lime is al much less expensive chemical than are sodium base compounds such as sodium carbonate, caustic soda, or The pulp may in such case be digested asv a 6% suspension in an 8% to 10% milk of lime liquor at 212"l F. for four hours. After digestion has been completed, the major portion of lime will have gone into solution, but there will be residual lime in suspension. The. liquor may be removed from the pulp, by Washing, it being preferable to use Wash water Ainitially rendered slightly acidic as with acetic acid, to ensure the removal of the lime.

'When digestion is effected in a lime liquor,

the smelt solution may be delivered in to a tank 15, wherein its sodium carbonate content may be causticized with lime. The causticized liquor may be used for the second digestion of the pulp, additional caustic soda being added thereto, if desired. After the second alkaline digestion of the pulp and Washing, it is. readily bleachable without suffering injury, as indicated on the drawing, the pulp being delivered into a tank 16, Whereln it may be bleached as ordinarily Withcalcium or sodium hypochlorite .bleach liquor. Preferably, bleaching is effected in a distinctly alkaline bleach liquor, this operation being carried out in several steps with washings between the steps, 'as this ensures no injury lbeing done to -the pulp.

A process such as 'hereinbefore described may also be advantageously practised in connection with the production of soda pulp. The smelt, liquor in a soda mill consists of a sodium carbonate solution 'and is ordinarily causticized. 'In accordance with the process vof the present invention, a portion4 of the sodium carbonate smelt solution may be used in lthe initial alkaline digestion of the pulp, thereby effecting a saving of the lime 'ordinarily used in causticization. The

rest of the smelt solution may be causticizedv as usual and employed for the second alkaline digestion. v

The process of the present invention hav- 1. A process which comprises treating raw cellulose pulp with a lignin-reactive reagent, Washing the pulp, digesting the pulp in a weakly alkaline liquorl to dissolve certain ligneous reaction products therefrom, and then digesting the pulp in a strongly alkaline liquor to dissolve other non-alpha cellulose components therefrom.

2. A process which .comprises treating raW cellulose pulp with a lignin-reactive reagent, digesting the pulp in a sodium carbonate solution to dissolve certain ligneous reaction products therefrom, and then digest-ing the pulp in a strongly alkaline liquor to remove other non-alpha cellulose components, therefrom.

3. A process which comprises chlorinating raW cellulose pulp, digesting vthe chlorinated pulp in a sodium carbonate solution to dissolve certain chlorine-lignin reaction vprodlucts therefrom, and then digesting the pulp in a caustic soda solution to dissolve other non-alpha cellulose components therefrom.

4. A process which lcomprises treating raw kraft pulp with chlorine Water, digesting the chlorinated pulp at elevated temperature in a sodium carbonate solution to dissolve certain chlorine-lignin reaction products therefrom, and then digesting the pulp at about room temperature in a strong solu-4 tion of caustic soda and sodium sulphide to dissolve other non-alpha cellulose components therefrom. 1

5. A process Which comprises cookingraw cellulosic material in a. cooking liquor containing sodium-sulphur compounds to effect fi er liberation, vseparating the spent liquorI from the resulting pulp, smelting the` inorganic constituent of the spent liquor to produce sodium carbonate and sodium sulphide, separating sodium carbonate from the rest of the smelt, digesting the pulp in a solution of the separated sodiumv carbonate,

and then digesting the pulp in a solution of,

the rest of the smelt.

6. A process which comprises cooking raw' ycelluloslc material in a cooking li uor containing sodium-sulphur compoun s to effect fiber libera-tion, separating the spent liquor from the resulting pulp, smelting the inorganic constituent of the spent liquor to4 i produce sodium 'carbonate and sodium sulphide, forming an aqueous smelt solution, crystallzmg sodium carbonate from the smelt solution, digesting. the pulp ina solution of the separated sodium carbonate, causticizing the restof the smelt solution,

and digesting the pulp in a solution of the causticized smelt solution.

7. A process which comprises cooking raw cellulosic material in a kraft liquor to effect fiber liberation, separating the spent liquor from the resulting pulp, smelting the inori ganic content ofthe spent liquor to produce signature.

a smelt containing sodium carbonate and sodium sulphide, selectively crystallizing sodium carbonate from the smelt solution, treating the rauT pulp withV a lignin-reactive reagent, digesting the pulp in a solution of the crystallized sodium carbonate, and then digestingthe pulp in the smelt mother solution.

8. A process which comprises digesting preliberated cellulose lpulp in a milk of lime liquor to dissolve non-alpha cellulose components soluble in such a liquor, and then digesting the pulp in a much stronger alkaline liquor to dissolve other non-alpha celluloser components therefrom.

9. A process which comprises digesting preliberated cellulose pulp in a milk of lime liquor to dissolve non-alpha cellulose com- `ponents soluble in such a liquor, Washing the pulp with Water initially in an acid condition to ensure the removal of lime, and then` digesting the pulp in a solution of caustic soda to dissolve other non-alpha cellulose components therefrom.

l0. A process which comprises digesting .preliberated cellulose vpulp in a sodium carbonate solution to dissolve non-alpha cellu- -lose components soluble in such a solution and then digesting Vthe pulp in a muchl stronger alkaline liquor to dissolve other non-alpha cellulose. components therefrom.

11. A process which comprisesl digesting .preliberated cellulose pulp in a sodium carbonate solution to dissolve non-alpha cellulose components soluble in such liquor, and then digesting the pulp in a caustic soda solution to dissolve other non-alpha cellulose components therefrom.

In testimony whereof I have aixed my GEORGE A. RICHTER. 

